For example, Japanese Laid-Open Patent Publication No. 9-327139 discloses a rotor having a consequent-pole structure as a rotor for a motor. The rotor of the above publication includes a rotor core, a plurality of magnets arranged along the circumferential direction of the rotor core, and a plurality of salient poles integrally formed with the rotor core. Each salient pole is located between a circumferentially adjacent pair of the magnets. The magnets function as either north poles or south poles, and the salient poles function as magnetic poles different from those of the magnets.
In a common rotor, in which all the magnetic poles are formed by magnets, magnetic flux flowing through a part of the rotor core close to the backside of each magnet is evenly divided at the center in the circumferential direction of the magnet to both sides in the circumferential direction. Accordingly, the rotor has a stable magnetic balance. In contrast, in the rotor having a consequent-pole structure of the above publication, the salient poles do not induce magnetic flux. Thus, depending on the ever-changing positional relationship between the salient poles and the teeth of the stator facing the salient poles, magnetic flux flowing through a part of the rotor core close to the backside of each magnet is not evenly divided between both sides in the circumferential direction from the circumferential center of the magnet. Instead, the magnetic flux flows in a greater amount through one of the salient poles at the circumferential ends of the magnet, which salient pole has a lower magnetic resistance. That is, since the directional property and the amount of magnetic flux at each salient pole change depending on the positional relationship between the salient pole and the teeth, the rotor become magnetically imbalanced. This deteriorates the rotational performance of the motor. Specifically, the torque may be lowered or the vibration may increase.